High voltage electrolyte additives

ABSTRACT

Described herein are additives for use in electrolytes that provide a number of desirable characteristics when implemented within batteries, such as high capacity retention during battery cycling at high temperatures. In some embodiments, a high voltage electrolyte includes a base electrolyte and one or more polymer additives, which impart these desirable performance characteristics. The polymer additives can be homopolymers or copolymers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application numberPCT/US2017/035709 filed on Jun. 2, 2017 entitled “High VoltageElectrolyte Additives”. The '709 international application claimspriority benefits from U.S. provisional patent application No.62/344,942 filed on Jun. 2, 2016 entitled “High Voltage ElectrolyteAdditives”. Both of these applications are hereby incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION

The present invention is in the field of battery technology and, moreparticularly, in the area of additive compounds for use with high-energyelectrodes in electrochemical cells.

A liquid electrolyte serves to transport ions between electrodes in abattery. Organic carbonate-based electrolytes are most commonly used inlithium-ion (“Li-ion”) batteries and, more recently, efforts have beenmade to develop new classes of electrolytes based on sulfones, silanes,and nitriles. Unfortunately, these conventional electrolytes typicallycannot be operated at high voltages, since they are unstable above 4.3 Vor other high voltages. At high voltages, conventional electrolytes candecompose, for example, by catalytic oxidation in the presence ofcathode materials, to produce undesirable products that affect both theperformance and safety of a battery. Conventional electrolytes may alsobe degraded by reduction by the anodes when the cells are charged.

As described in more detail below, solvents, salts, or additives havebeen incorporated into the electrolyte to decompose on the electrode toform a protective film called a solid electrolyte interphase (SEI).Depending on the exact chemical system, this film can be composed oforganic or inorganic lithium salts, organic molecules, oligomers, orpolymers. Often, several components of the electrolyte are involved inthe formation of the SEI (e.g., lithium salt, solvent, and additives).As a result, depending on the rate of decomposition of the differentcomponents, the SEI can be more or less homogenous.

In past research, organic compounds containing polymerizable functionalgroups such as alkenes, furan, thiophene, and pyrrole had been reportedto form an SEI on the cathode of lithium ion batteries. See, e.g., Y.-S.Lee et al., Journal of Power Sources 196 (2011) 6997-7001. Theseadditives likely undergo polymerization during cell charging to formpassivation films on the electrodes. SEIs are known to contain highmolecular weight species. However, in situ polymerization during theinitial charge often cannot be controlled in a precise enough manner toprevent non-uniform SEIs comprised of polymer or oligomer mixtures witheither heterogeneous molecular weight, heterogeneous composition, oreven undesired adducts. The non-uniformity of the SEI often results inpoor mechanical and electrochemical stability, which is believed to be amain cause of cycle life degradation in lithium ion batteries. Thus, theimprovement in cell performance using these materials is limited.

Further, certain organic polymers have also been used as solidelectrolytes for lithium ion batteries due to the generally lowvolatility and safety of polymeric molecules as compared to smallerorganic molecules, such as organic carbonates. However, practicalapplication of such systems has been limited due to poor ionicconductivity.

For high-energy cathode materials, electrolyte stability remains achallenge. Recently, the need for better performance and higher capacitylithium ion secondary batteries used for power sources is dramaticallyincreasing. Lithium transition metal oxides such as LiCoO₂ (“LCO”) andLiNi_(0.33)Mn_(0.33)Co_(0.33)O₂ (“NMC”) are state-of-the-art high-energycathode materials used in commercial batteries. Yet only about 50% ofthe theoretical capacity of LCO or NMC cathodes can be used with stablecycle life. To obtain the higher capacity, batteries containing thesehigh-energy materials need to be operated at higher voltages, such asvoltages up to about 4.7V. However, above about 4.3V, conventionalelectrolytes degrade and this leads to a significant deterioration ofthe cycle life. Further, the decomposition of the electrolyte at highervoltages can generate gas (such as CO₂, O₂, ethylene, H₂) and acidicproducts, both of which can damage a battery. These effects are furtherenhanced in “high nickel” NMC compositions such asLiNi_(0.6)Mn_(0.2)Co_(0.2)O₂ or LiNi_(0.8)Mn_(0.1)Co_(0.1)O₂ or otherswhich can provide higher capacities due to the electrochemistry of thenickel.

Many of these same challenges occur when a battery is operated at hightemperature. That is, conventional electrolytes can decompose byoxidation or may be degraded by reduction at high temperature analogousto the way these mechanisms affect the electrolytes at high voltage.Other parasitic reactions can also occur at elevated temperature.

As disclosed herein, these challenges and others are addressed in highenergy lithium ion secondary batteries including cathode activematerials that are capable of operation at high voltage.

BRIEF SUMMARY OF THE INVENTION

Certain embodiments relate to a battery including an anode, a cathode,and an electrolyte formulation including a lithium salt, a non-aqueoussolvent, and a homopolymer additive compound. In some embodiments theelectrolyte formulation includes a homopolymer additive polymerized froma maleic anhydride monomer.

Other embodiments relate to a battery including an anode, a cathode, andan electrolyte including a lithium salt, a non-aqueous solvent, and acopolymer additive compound. In some embodiments the electrolyteformulation includes a copolymer additive polymerized from a maleicanhydride monomer and a vinyl substituted aromatic monomer.

The cathode material can be an NMC material.

Certain embodiments include methods making, using, and conditioning suchbatteries for use.

Other aspects and embodiments of the invention are also contemplated.The foregoing summary and the following detailed description are notmeant to restrict the invention to any particular embodiment but aremerely meant to describe some embodiments of the invention.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a lithium ion battery implemented in accordance withan embodiment of the invention.

FIG. 2 illustrates the operation of a lithium ion battery and agraphical representation of an illustrative non-limiting mechanism ofaction of an electrolyte including an additive compound, according to anembodiment of the invention.

FIG. 3 illustrates the copolymer structure of additives according tocertain embodiments of the invention.

FIG. 4 illustrates the high temperature cycle life testing of anelectrolyte formulation according to certain embodiments of theinvention.

FIG. 5 illustrates the high temperature cycle life testing of anelectrolyte formulation according to certain embodiments of theinvention.

FIG. 6 illustrates the high temperature cycle life testing of anelectrolyte formulation according to certain embodiments of theinvention.

FIG. 7 illustrates the high temperature cycle life testing of anelectrolyte formulation according to certain embodiments of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions apply to some of the aspects described withrespect to some embodiments of the invention. These definitions maylikewise be expanded upon herein. Each term is further explained andexemplified throughout the description, figures, and examples. Anyinterpretation of the terms in this description should take into accountthe full description, figures, and examples presented herein.

The singular terms “a,” “an,” and “the” include the plural unless thecontext clearly dictates otherwise. Thus, for example, reference to anobject can include multiple objects unless the context clearly dictatesotherwise.

The terms “substantially” and “substantial” refer to a considerabledegree or extent. When used in conjunction with an event orcircumstance, the terms can refer to instances in which the event orcircumstance occurs precisely as well as instances in which the event orcircumstance occurs to a close approximation, such as accounting fortypical tolerance levels or variability of the embodiments describedherein.

The term “about” refers to the range of values approximately near thegiven value in order to account for typical tolerance levels,measurement precision, or other variability of the embodiments describedherein.

The term “specific capacity” refers to the amount (e.g., total ormaximum amount) of electrons or lithium ions a material is able to hold(or discharge) per unit mass and can be expressed in units of mAh/g. Incertain aspects and embodiments, specific capacity can be measured in aconstant current discharge (or charge) analysis, which includesdischarge (or charge) at a defined rate over a defined voltage rangeagainst a defined counter electrode. For example, specific capacity canbe measured upon discharge at a rate of about 0.05 C (e.g., about 8.75mA/g) from 4.45 V to 3.0 V versus a Li/Li⁺ counter electrode. Otherdischarge rates and other voltage ranges also can be used, such as arate of about 0.1 C (e.g., about 17.5 mA/g), or about 0.5 C (e.g., about87.5 mA/g), or about 1.0 C (e.g., about 175 mA/g).

A rate “C” refers to either (depending on context) the discharge currentas a fraction or multiple relative to a “1 C” current value under whicha battery (in a substantially fully charged state) would substantiallyfully discharge in one hour, or the charge current as a fraction ormultiple relative to a “1 C” current value under which the battery (in asubstantially fully discharged state) would substantially fully chargein one hour.

The term “coulombic efficiency” is sometimes abbreviated herein as CEand refers the efficiency with which charge is transferred in a givencycle.

The term “rated charge voltage” refers to an upper end of a voltagerange during operation of a battery, such as a maximum voltage duringcharging, discharging, and/or cycling of the battery. In some aspectsand some embodiments, a rated charge voltage refers to a maximum voltageupon charging a battery from a substantially fully discharged statethrough its (maximum) specific capacity at an initial cycle, such as the1st cycle, the 2nd cycle, or the 3rd cycle. In some aspects and someembodiments, a rated charge voltage refers to a maximum voltage duringoperation of a battery to substantially maintain one or more of itsperformance characteristics, such as one or more of coulombicefficiency, retention of specific capacity, retention of energy density,and rate capability.

The term “rated cut-off voltage” refers to a lower end of a voltagerange during operation of a battery, such as a minimum voltage duringcharging, discharging, and/or cycling of the battery. In some aspectsand some embodiments, a rated cut-off voltage refers to a minimumvoltage upon discharging a battery from a substantially fully chargedstate through its (maximum) specific capacity at an initial cycle, suchas the 1st cycle, the 2nd cycle, or the 3rd cycle, and, in such aspectsand embodiments, a rated cut-off voltage also can be referred to as arated discharge voltage. In some aspects and some embodiments, a ratedcut-off voltage refers to a minimum voltage during operation of abattery to substantially maintain one or more of its performancecharacteristics, such as one or more of coulombic efficiency, retentionof specific capacity, retention of energy density, and rate capability.

The “maximum voltage” refers to the voltage at which both the anode andthe cathode are fully charged. In an electrochemical cell, eachelectrode may have a given specific capacity and one of the electrodeswill be the limiting electrode such that one electrode will be fullycharged and the other will be as fully charged as it can be for thatspecific pairing of electrodes. The process of matching the specificcapacities of the electrodes to achieve the desired capacity of theelectrochemical cell is “capacity matching.”

The term “NMC” refers generally to cathode materials containingLiNi_(x)Mn_(y)Co_(z)O_(w) and includes, but is not limited to, cathodematerials containing LiNi_(0.33)Mn_(0.33)Co_(0.33)O₂ andLiNi_(0.5)Mn_(0.3)Co_(0.2)O₂.

The term “polymer” refers generally to a molecule whose structure iscomposed of multiple repeating units. The structure can be linear orbranched. In the chemical formulas depicted herein, the subscripts “m”,“n” and “o” refer to the number of repeating units and are positiveintegers.

The term “homopolymer” refers to a polymer that is made bypolymerization of a single monomer.

The term “copolymer” refers generally to a molecule whose structure iscomposed of at least two different repeating units. The structure can bealternating, periodic, statistical, random, block, linear, branched,combinations thereof, or other structure. In certain embodimentsdisclosed herein, the copolymer is preferably a block copolymer. Incertain embodiments disclosed herein, the copolymer is preferably arandom copolymer. In certain embodiments disclosed herein, the copolymeris preferably a branched copolymer.

As used herein, the term “moiety” refers to a distinct, structurallyidentifiable, structurally isolated, or structurally named portion of amolecule.

As used herein, the term “alkane” refers to a saturated hydrocarbon,including the more specific definitions of “alkane” herein. For certainembodiments, an alkane can include from 1 to 100 carbon atoms. The term“lower alkane” refers to an alkane that includes from 1 to 20 carbonatoms, such as from 1 to 10 carbon atoms, while the term “upper alkane”refers to an alkane that includes more than 20 carbon atoms, such asfrom 21 to 100 carbon atoms. The term “branched alkane” refers to analkane that includes one or more branches, while the term “unbranchedalkane” refers to an alkane that is straight-chained. The term“cycloalkane” refers to an alkane that includes one or more ringstructures. The term “heteroalkane” refers to an alkane that has one ormore of its carbon atoms replaced by one or more heteroatoms, such as N,Si, S, O, F, and P. The term “substituted alkane” refers to an alkanethat has one or more of its hydrogen atoms replaced by one or moresubstituent groups, such as halo groups, while the term “unsubstitutedalkane” refers to an alkane that lacks such substituent groups.Combinations of the above terms can be used to refer to an alkane havinga combination of characteristics. For example, the term “branched loweralkane” can be used to refer to an alkane that includes from 1 to 20carbon atoms and one or more branches. Examples of alkanes includemethane, ethane, propane, cyclopropane, butane, 2-methylpropane,cyclobutane, and hetero or substituted forms thereof.

As used herein, the term “alkyl group” refers to a monovalent form of analkane. For example, an alkyl group can be envisioned as an alkane withone of its hydrogen atoms removed to allow bonding to another group.Alkyls include lower alkyls (an alkyl that includes from 2 to 20 carbonatoms, such as from 2 to 10 carbon atoms), upper alkyls (an alkyl thatincludes more than 20 carbon atoms, such as from 21 to 100 carbonatoms), cycloalkyls (an alkyl that includes one or more ringstructures), heteroalkyls (an alkyl that has one or more of its carbonatoms replaced by one or more heteroatoms, such as N, Si, S, O, F, andP), and branched forms of all such alkyls. Alkyls can be substitutedsuch that one or more of its hydrogen atoms is replaced by one or moresubstituent groups, such as halo groups. An alkyl can have a combinationof characteristics. For example, a substituted lower alkyl can refer toan alkyl. Examples of alkyl groups include methyl, ethyl, n-propyl,isopropyl, cyclopropyl, butyl, isobutyl, t-butyl, cyclobutyl, andhetero, or substituted forms thereof.

As used herein, “aromatic” refers to both arenes and aryls. The term“arene” refers to an aromatic hydrocarbon. For certain embodiments, anarene can include from 5 to 100 carbon atoms, which encompasses lowerarenes (an arene that includes from 5 to 20 carbon atoms) and upperarenes (an arene that includes more than 20 carbon atoms, such as from21 to 100 carbon atoms). Arenes can be monocyclic and includes a singlearomatic ring structure or polycyclic arene and includes more than onearomatic ring structure. Arenes include heteroarenes (an arene that hasone or more of its carbon atoms replaced by one or more heteroatoms,such as N, Si, S, O, F, and P) and substituted arenes (an arene that hasone or more of its hydrogen atoms replaced by one or more substituentgroups, such as alkyl groups, alkenyl groups, alkynyl groups, halogroups, hydroxy groups, alkoxy groups, alkenoxy groups, alkynoxy groups,aryloxy groups, carboxy groups, cyano groups, nitro groups, aminogroups, N-substituted amino groups, silyl groups, and siloxy groups.Combinations of the above terms can be used to refer to an arene havinga combination of characteristics. Examples of arenes include benzene,biphenyl, naphthalene, anthracene, pyridine, pyridazine, pyrimidine,pyrazine, quinoline, isoquinoline, and charged, hetero, or substitutedforms thereof.

The term “aryl group” refers to a monovalent form of an arene. An arylcan include from 5 to 100 carbon atoms, which encompasses lower aryls(an aryl that includes from 5 to 20 carbon atoms) and upper aryls (anaryl that includes more than 20 carbon atoms, such as from 21 to 100carbon atoms). Aryls can be monocyclic and includes a single aromaticring structure or polycyclic aryl and includes more than one aromaticring structure. Aryls include heteroaryls (an aryl that has one or moreof its carbon atoms replaced by one or more heteroatoms, such as N, Si,S, O, F, and P) and substituted aryls (an aryl that has one or more ofits hydrogen atoms replaced by one or more substituent groups, such asalkyl groups, alkenyl groups, alkynyl groups, halo groups, hydroxygroups, alkoxy groups, alkenoxy groups, alkynoxy groups, aryloxy groups,carboxy groups, cyano groups, nitro groups, amino groups, N-substitutedamino groups, silyl groups, and siloxy groups. Combinations of the aboveterms can be used to refer to an aryl having a combination ofcharacteristics. Examples of aryl groups include phenyl, biphenylyl,naphthyl, pyridinyl, pyridazinyl, pyrimidinyl, pyrazinyl, quinolyl,isoquinolyl, and charged, hetero, or substituted forms thereof.

To the extent certain battery characteristics can vary with temperature,such characteristics are specified at room temperature (about 30 degreesC.), unless the context clearly dictates otherwise.

Ranges presented herein are inclusive of their endpoints. Thus, forexample, the range 1 to 3 includes the values 1 and 3 as well asintermediate values.

FIG. 1 illustrates a lithium ion battery 100 implemented in accordancewith an embodiment of the invention. The battery 100 includes an anode102, a cathode 106, and a separator 108 that is disposed between theanode 102 and the cathode 106. In the illustrated embodiment, thebattery 100 also includes a high voltage electrolyte 104, which isdisposed within and between the anode 102 and the cathode 106 andremains stable during high voltage battery cycling.

The operation of the battery 100 is based upon reversible intercalationand de-intercalation of lithium ions into and from host materials of theanode 102 and the cathode 106. Other implementations of the battery 100are contemplated, such as those based on conversion chemistry. Referringto FIG. 1, the voltage of the battery 100 is based on redox potentialsof the anode 102 and the cathode 106, where lithium ions areaccommodated or released at a lower potential in the former and a higherpotential in the latter. To allow both a higher energy density and ahigher voltage platform to deliver that energy, the cathode 106 includesan active cathode material for high voltage operations at or above 4.3V.

Examples of suitable high voltage cathode materials include phosphates,fluorophosphates, fluorosulfates, fluorosilicates, spinels, lithium-richlayered oxides, and composite layered oxides. Further examples ofsuitable cathode materials include: spinel structure lithium metaloxides, layered structure lithium metal oxides, lithium-rich layeredstructured lithium metal oxides, lithium metal silicates, lithium metalphosphates, metal fluorides, metal oxides, sulfur, and metal sulfides.Examples of suitable anode materials include conventional anodematerials used in lithium ion batteries, such as lithium, graphite(“LixC₆”), and other carbon, silicate, or oxide-based anode materials.

FIG. 2 illustrates operation of a lithium ion battery and anillustrative, non-limiting mechanism of action of an improvedelectrolyte, according to an embodiment of the invention. Without beingbound by a particular theory not recited in the claims, the inclusion ofone or more stabilizing additive compounds in an electrolyte solutioncan, upon operation of the battery (e.g., during conditioning thereof),passivate a high voltage cathode material, thereby reducing orpreventing reactions between bulk electrolyte components and the cathodematerial that can degrade battery performance.

Referring to FIG. 2, an electrolyte 202 includes a base electrolyte,and, during initial battery cycling, components within the baseelectrolyte can assist in the in-situ formation of a protective film (inthe form of a solid electrolyte interface (“SEI”) 206) on or next to ananode 204. The anode SEI 206 can inhibit reductive decomposition of thehigh voltage electrolyte 202. Preferably, and without being bound bytheory not recited in the claims, for operation at voltages at or above4.2 V, the electrolyte 202 can also include additives that can assist inthe in-situ formation of a protective film (in the form of a SEI 208 oranother derivative) on or next to a cathode 200. The cathode SEI 208 caninhibit oxidative decomposition of the high voltage electrolyte 202 thatcan otherwise occur during high voltage operations. As such, the cathodeSEI 208 can inhibit oxidative reactions in a counterpart manner to theinhibition of reductive reactions by the anode SEI 206. In theillustrated embodiment, the cathode SEI 208 can have a thickness in thesub-micron range, and can include one or more chemical elementscorresponding to, or derived from, those present in one or moreadditives, such as silicon or other heteroatom included in one or moreadditives. Advantageously, one or more additives can preferentiallypassivate the cathode 200 and can selectively contribute towards filmformation on the cathode 200, rather than the anode 204. Suchpreferential or selective film formation on the cathode 200 can impartstability against oxidative decomposition, with little or no additionalfilm formation on the anode 204 (beyond the anode SEI 206) that canotherwise degrade battery performance through resistive losses. Moregenerally, one or more additives can decompose below a redox potentialof the cathode material and above a redox potential of SEI formation onthe anode 204.

Without being bound by a particular theory not recited in the claims,the formation of the cathode SEI 208 can occur through one or more ofthe following mechanisms: (1) the additive compound(s) can decompose toform the cathode SEI 208, which inhibits further oxidative decompositionof electrolyte components; (2) the additive compound(s) or itsdecomposed product(s) form or improve the quality of a passivation filmon the cathode or anode; (3) the additive compounds can form anintermediate product, such as a complex with LiPF₆ or a cathodematerial, which intermediate product then decomposes to form the cathodeSEI 208 that inhibits further oxidative decomposition of electrolytecomponents; (4) the additive compounds can form an intermediate product,such as a complex with LiPF₆, which then decomposes during initialcharging. The resulting decomposition product can then further decomposeduring initial charging to form the cathode SEI 208, which inhibitsfurther oxidative decomposition of electrolyte components; (5) theadditive compounds can stabilize the cathode material by preventingmetal ion dissolution.

Other mechanisms of action of the electrolyte 202 are contemplated,according to an embodiment of the invention. For example, and in placeof, or in combination with, forming or improving the quality of thecathode SEI 208, one or more additives or a derivative thereof (e.g.,their decomposition product) can form or improve the quality of theanode SEI 206, such as to reduce the resistance for lithium iondiffusion through the anode SEI 206. As another example, one or moreadditives or a derivative thereof (e.g., their decomposition product)can improve the stability of the electrolyte 202 by chemically reactingor forming a complex with other electrolyte components. As a furtherexample, one or more additives or a derivative thereof (e.g., theirdecomposition product) can scavenge decomposition products of otherelectrolyte components or dissolved electrode materials in theelectrolyte 202 by chemical reaction or complex formation. Any one ormore of the cathode SEI 208, the anode SEI 206, and the otherdecomposition products or complexes can be viewed as derivatives, whichcan include one or more chemical elements corresponding to, or derivedfrom, those present in one or more additives, such as a heteroatomincluded in the additives.

Certain embodiments are related to a class of polymeric additives fornon-aqueous electrolytes. Such embodiments include several electrolyteadditives that improve the oxidative stability of the electrolyte andthe cycle life and coulombic efficiency of electrochemical cellscontaining these additives.

A high voltage electrolyte according to some embodiments of theinvention can be formed with reference to the formula:base electrolyte+additive→high voltage electrolyte  (1)

A high temperature electrolyte according to some embodiments of theinvention can be formed with reference to the formula:base electrolyte+additive→high temperature electrolyte  (2)

In formulas (1) and (2), the base electrolyte can include one or moresolvents and one or more salts, such as lithium-containing salts in thecase of lithium ion batteries. Examples of suitable solvents includenonaqueous electrolyte solvents for use in lithium ion batteries,including carbonates, such as ethylene carbonate, dimethyl carbonate,ethyl methyl carbonate, propylene carbonate, methyl propyl carbonate,and diethyl carbonate; sulfones; silanes; nitriles; esters; ethers; andcombinations thereof. The base electrolyte can also include additionalsmall molecule additives.

Referring to formulas (1) and (2), an amount of a particular additivecan be expressed in terms of a weight percent of the additive relativeto a total weight of the electrolyte solution (or wt. %). For example,an amount of an additive can be in the range of about 0.01 wt. % toabout 30 wt. %, such as from about 0.05 wt. % to about 30 wt. %, fromabout 0.01 wt. % to about 20 wt. %, from about 0.2 wt. % to about 15 wt.%, from about 0.2 wt. % to about 10 wt. %, from about 0.2 wt. % to about5 wt. %, or from about 0.2 wt. % to about 1 wt. %, and, in the case of acombination of multiple additive, a total amount of the additive can bein the range of about 0.01 wt. % to about 30 wt. %, such as from about0.05 wt. % to about 30 wt. %, from about 0.01 wt. % to about 20 wt. %,from about 0.2 wt. % to about 15 wt. %, from about 0.2 wt. % to about 10wt. %, from about 0.2 wt. % to about 5 wt. %, or from about 0.2 wt. % toabout 1 wt. %. An amount of an additive also can be expressed in termsof a ratio of the number of moles of the additive per unit surface areaof either, or both, electrode materials. For example, an amount of acompound can be in the range of about 10⁻⁷ mol/m² to about 10⁻² mol/m²,such as from about 10⁻⁷ mol/m² to about 10⁻⁵ mol/m², from about 10⁻⁵mol/m² to about 10⁻³ mol/m², from about 10⁻⁶ mol/m² to about 10⁻⁵mol/m², or from about 10⁻⁴ mol/m² to about 10⁻² mol/m². As furtherdescribed below, a additive can be consumed or can react, decompose, orundergo other modifications during initial battery cycling. As such, anamount of a compound can refer to an initial amount of the compound usedduring the formation of the electrolyte solutions according to formulas(1) or (2), or can refer to an initial amount of the additive within theelectrolyte solution prior to battery cycling (or prior to anysignificant amount of battery cycling).

Resulting performance characteristics of a battery can depend upon theidentity of a particular additive used to form the high voltageelectrolyte according to formulas (1) or (2), an amount of the compoundused, and, in the case of a combination of multiple compounds, arelative amount of each compound within the combination. Accordingly,the resulting performance characteristics can be fine-tuned or optimizedby proper selection of the compounds and adjusting amounts of thecompounds in formulas (1) or (2).

The formation according to formulas (1) or (2) can be carried out usinga variety of techniques, such as by mixing the base electrolyte and theadditives, dispersing the additives within the base electrolyte,dissolving the additives within the base electrolyte, or otherwiseplacing these components in contact with one another. The additives canbe provided in a liquid form, a powdered form (or another solid form),or a combination thereof. The additives can be incorporated in theelectrolyte solutions of formulas (1) or (2) prior to, during, orsubsequent to battery assembly.

The electrolyte solutions described herein can be used for a variety ofbatteries containing a high voltage cathode or a low voltage cathode,and in batteries operated at high temperatures. For example, theelectrolyte solutions can be substituted in place of, or used inconjunction with, conventional electrolytes for lithium ion batteriesfor operations at or above 4.3 V. In particular, these additives areuseful for lithium ion batteries containing NMC cathode materials.

Batteries including the electrolyte solutions can be conditioned bycycling prior to commercial sale or use in commerce. Such conditioningcan include, for example, providing a battery, and cycling such batterythrough at least 1, at least 2, at least 3, at least 4, or at least 5cycles, each cycle including charging the battery and discharging thebattery at a rate of 0.05 C (e.g., a current of 8.75 mA/g) between 4.45Vand 3.0V (or another voltage range) versus a reference counterelectrode, such as a graphite anode. Charging and discharging can becarried out at a higher or lower rate, such as at a rate of 0.1 C (e.g.,a current of 17.5 mA/g), at a rate of 0.5 C (e.g., a current of 87.5mA/g), or at a rate of 1 C (e.g., a current of 175 mA/g). Typically abattery is conditioned with 1 cycle by charging at 0.05 C rate to 4.45Vfollowed by applying constant voltage until the current reaches 0.02 C,and then discharging at 0.05 C rate to 3V.

The polymer and copolymer additives according to embodiments herein aremolecules formed from numerous repeated monomer units, as isconventionally understood in the art. Such polymer and copolymeradditives contain certain functional groups along the backbone of thepolymer chain.

The copolymers disclosed herein are generally referred to by the namesof the monomer molecules that are used to synthesize the copolymer. Forexample, a polymers synthesized with a maleic anhydride monomer isreferred to as poly(maleic anhydride) even though the repeating unit inthe backbone of the formed polymer appears to be a succinic anhydride.Thus, while the copolymers referred to herein generally are named by themonomers used to form them, it is possible that some of the copolymerscould be referred to by alternate names. The disclosure is intended toencompass such variations in chemical nomenclature without departingfrom the scope and spirit of the invention.

The polymer additives disclosed herein are all homopolymers or random,block, or branched co-polymers containing a maleic anhydride functionalgroup. The homopolymers include monomer units that are synthesized frommaleic anhydride or substituted maleic anhydride. The random, block, orbranched copolymers are synthesized from polymeric reactions usingmaleic anhydride or with other polymerizable precursors, such asalkenes, aromatics, maleic imides, unsaturated esters, unsaturatedacids, unsaturated ethers, furans, and ethylene glycols.

FIG. 3 illustrates the copolymer structure of additives according tocertain embodiments of the invention. The “A” monomer is preferablymaleic anhydride. The “B” monomer and the “C” monomer can be any ofseveral monomers where the monomer includes alkene, aromatic, maleicimide, unsaturated ester, unsaturated acid, unsaturated ether, furan,and ethylene glycol functionality. The value of m is greater than 1; andis in some cases greater than 100; in some cases greater than 250; insome cases greater than 500; in some cases greater than 1,000; in somecases greater than 5,000; in some cases greater than 10,000; in somecases greater than 50,000; in some cases greater than 100,000; in somecases greater than 500,000; and in some cases greater than 1,000,000.The value of n can be 0, but also can be greater than 0; and is in somecases greater than 100; in some cases greater than 250; in some casesgreater than 500; in some cases greater than 1,000; in some casesgreater than 5,000; in some cases greater than 10,000; in some casesgreater than 50,000; in some cases greater than 100,000; in some casesgreater than 500,000; and in some cases greater than 1,000,000. Thevalue of o can be 0, but also can be greater than 0; and is in somecases greater than 100; in some cases greater than 250; in some casesgreater than 500; in some cases greater than 1,000; in some casesgreater than 5,000; in some cases greater than 10,000; in some casesgreater than 50,000; in some cases greater than 100,000; in some casesgreater than 500,000; and in some cases greater than 1,000,000.

The random, block, or branched copolymers additives of certainembodiments of the invention provide further performance improvements ascompared to the homopolymer embodiments for at least the followingreasons: (i) improved mechanical properties of certain random, block, orbranched copolymer additives because of cross-linking of poly(maleicanhydride) units to other polymeric units; and (ii) improved chemicalproperty of these co-polymers can also be obtained by introducing theadditional functional group into the polymeric structures

In certain embodiments, the homopolymer additives disclosed herein aresynthesized from a maleic anhydride monomer. The poly(maleic anhydride)homopolymer is represented by Formula (a):

where n represents the number of repeat units in the polymer.

In certain embodiments, the random, block, or branched copolymer cancontain a monomer that contains aromatic functionality. In particular,substituted aromatics in which the substituted groups are vinyl groups.Examples of vinyl substituted aromatics are presented herein asnon-limiting examples of the group of substituted aromatics.

Certain properties are preferred in copolymer additives for use inelectrochemical cells. For example, the additives preferably are: (i)either chemically resistant to oxidation and/or reduction under the cellconditions or, if not chemically resistant to oxidation and/orreduction, then the additives should decompose to intermediates orproducts that form a stable SEI film on the anode, cathode, or both; and(ii) sufficiently soluble in electrolyte solution at room temperatureand to make the electrolyte solution viscosity during battery operationnot worse than without the additive.

These random, block, or branched copolymer additives are soluble in theconventional electrolyte solution and have functional groups thatcontribute to stable and uniform SEI formation. These random, block, orbranched copolymer additives can form more mechanically and chemicallystable SEI films compared with molecular or short-chain oligomers orpolymer additives.

In addressing the challenges of high energy cathode materials, theadditives according to embodiments disclosed herein have a number ofbenefits, including: (i) unique functional groups pre-arranged in thebackbone, which allows the random, block, or branched copolymeradditives to strongly and evenly adsorb on to the surface of theelectrodes before decomposition and potentially improving the qualityand stability of the resulting SEI; and (ii) mechanical and chemicalstability as compared to organic oligomers and short-chain polymersformed from conventional solvents and additives due to the pre-formedcopolymer backbone.

Further, the high molecular weight SEI species resulting from reactionsof the ring-opening decomposition of the succinic anhydride moiety withlithium salt species can: (i) be homogenously dispersed throughout theSEI to form a more uniform film; (ii) provide a more mechanically andchemically stable SEI on both cathode and anode surface; (iii) be usedto chelate cathode transition metal ions dissolved in the electrolyte,which prevents anode SEI breakdown leading to capacity fade; and (iv)function as scavenger or acidic reactive species and/or protonicreactive species, which decreases chain reactions of solvent and SEIdecomposition caused by those reactive species.

In certain embodiments of the invention, the additive is present at anamount that is significantly lower than the amount of electrolyte saltpresent in the electrolyte formulation of the electrochemical cell. Theamount of additive can be expressed as a weight percent (wt %) of thetotal weight of the electrolyte formulation. In certain embodiments ofthe invention, the concentration of additive in the electronicformulation is less than or equal to the concentration at which theadditive would be at the saturation point in the electrolyte solvent. Incertain embodiments of the invention, the concentration of additive inthe electronic formulation is less than or equal to about 10 weightpercent, more preferably less than or equal to about 9 weight percent,more preferably less than or equal to about 8 weight percent, morepreferably less than or equal to about 7 weight percent, more preferablyless than or equal to about 6 weight percent, more preferably less thanor equal to about 5 weight percent, more preferably less than or equalto about 4 weight percent, more preferably less than or equal to about 3weight percent, and still more preferably less than or equal to about 2weight percent.

In certain embodiments of the invention, the concentration of eachadditive in the electronic formulation is equal to about 10.0 wt %, 9.9wt %, 9.8 wt %, 9.7 wt %, 9.6 wt %, 9.5 wt %, 9.4 wt %, 9.3 wt %, 9.2 wt%, 9.1 wt %, 9.0 wt %, 8.9 wt %, 8.8 wt %, 8.7 wt %, 8.6 wt %, 8.5 wt %,8.4 wt %, 8.3 wt %, 8.2 wt %, 8.1 wt %, 8.0 wt %, 7.9 wt %, 7.8 wt %,7.7 wt %, 7.6 wt %, 7.5 wt %, 7.4 wt %, 7.3 wt %, 7.2 wt %, 7.1 wt %,7.0 wt %, 6.9 wt %, 6.8 wt %, 6.7 wt %, 6.6 wt %, 6.5 wt %, 6.4 wt %,6.3 wt %, 6.2 wt %, 6.1 wt %, 6.0 wt %, 5.9 wt %, 5.8 wt %, 5.7 wt %,5.6 wt %, 5.5 wt %, 5.4 wt %, 5.3 wt %, 5.2 wt %, 5.1 wt %, 5.0 wt %,4.9 wt %, 4.8 wt %, 4.7 wt %, 4.6 wt %, 4.5 wt %, 4.4 wt %, 4.3 wt %,4.2 wt %, 4.1 wt %, 4.0 wt %, 3.9 wt %, 3.8 wt %, 3.7 wt %, 3.6 wt %,3.5 wt %, 3.4 wt %, 3.3 wt %, 3.2 wt %, 3.1 wt %, 3.0 wt %, 2.9 wt %,2.8 wt %, 2.7 wt %, 2.6 wt %, 2.5 wt %, 2.4 wt %, 2.3 wt %, 2.2 wt %, or2.1 wt %, 2.0 wt %, 1.9 wt %, 1.8 wt %, 1.7 wt %, 1.6 wt %, 1.5 wt %,1.4 wt %, 1.3 wt %, 1.2 wt %, 1.1 wt %, 1.0 wt %, 0.9 wt %, 0.8 wt %,0.7 wt %, 0.6 wt %, 0.5 wt %, 0.4 wt %, 0.3 wt %, 0.2 wt %, or 0.1 wt %.In certain embodiments of the invention, the concentration of additivein the electrolyte formulation is in the range of about 2.0 wt % toabout 0.5 wt %.

The following examples and methods describe specific aspects of someembodiments of the invention to illustrate and provide a description forthose of ordinary skill in the art. The examples and methods should notbe construed as limiting the invention, as the examples and methodsmerely provide specific methodology useful in understanding andpracticing some embodiments of the invention.

EXAMPLES

The homopolymers and the random, block, or branched copolymer additivesdisclosed herein can be described by identifying the monomer units usedto synthesize them. The examples below include routine chemicalmodifications of these listed monomers provided that the modificationsdo not substantially diminish the desired properties of the homopolymersor the random, block, or branched copolymers or substantially interferewith their performance as additives.

In an example of a random, block, or branched copolymer can contain amonomer that contains aromatic functionality and/or a monomer thatcontains alkene functionality, the monomer represented by Formula (b)can be polymerized with maleic anhydride:

Other examples of vinyl substituted aromatic monomers are represented byFormulas (c) and (d), and each can be polymerized with maleic anhydride:

Isopropyl benzene is an additional example of a substituted aromaticmonomers is represented by Formula (e) and it can be polymerized withmaleic anhydride:

A block copolymer according to certain embodiments that include aromaticmonomers is poly(styrene-co-maleic anhydride), represented by Formula(f):

Methods

Battery Cell Assembly.

Battery cells were formed in a high purity Argon filled glove box(M-Braun, O₂ and humidity content <0.1 ppm). In the case of the cathode,a commercial LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ (referred to herein as NMC532) or LiNi_(0.4)Mn_(0.4)Co_(0.2)O₂ (referred to herein as NMC 442)cathode material was mixed with dry poly(vinylidene fluoride), carbonblack powder, and liquid 1-methyl-2-pyrrolidinone to form a slurry. Theresulting slurry was deposited on an aluminum current collector anddried to form a composite cathode film. In the case of the anode, agraphitic carbon was mixed with dry poly(vinylidene fluoride), carbonblack powder, and liquid 1-methyl-2-pyrrolidinone to form a slurry. Theresulting slurry was deposited on a copper current collector and driedto form a composite anode film. Each battery cell included the compositecathode film, a polypropylene separator, and composite anode film. Aconventional electrolyte formed from 1M of LiPF6 in ethylene carbonateand ethyl methyl carbonate (EC:EMC=1:2) by volume was mixed with thedesired weight percentage of an embodiment of the inventive additive andadded to the battery cell. The battery cell was sealed and initiallycycled at ambient temperature using 0.1 C charge to upper cutoff voltage(up to 4.4V) followed by constant voltage hold until the current droppedto 0.05 C and then discharged to 2.8V using 0.1 C constant current. Thecycle was repeated one more time prior to high temperature cycling.

High Temperature Testing.

Test batteries were cycled up to 4.4V in an environment at a temperatureof about 40 degrees Celsius using 0.5 C charge followed by constantvoltage hold for 1 hour and then discharged to 2.8V using 0.5 C constantcurrent.

Table 1 shows certain data for the cycle life testing of someembodiments of the additives disclosed herein as compared to control andFIGS. 4 through 7 show the full cycle life testing. Specifically, datawas collected on the homopolymer additive poly(maleic anhydride) (“PMA”)with molecular weight around 10,000 and the block copolymer additivepoly(styrene-co-maleic anhydride) (“PS-co-PMA”) with molecular weightaround 9,500.

TABLE 1 Summary of additive performance compared to the controlelectrolyte 200th Cycle 1st Cycle 1st Cycle Capacity Capacity, 30° C. CERetention, 40° C. Additives Cell Chemistry (mAh/g) (%) (%,) NMC532 4.4 V4.4 V NMC 532/ 192 89 14.1% Control Graphite 0.5% PMA 4.4 V NMC 532/ 19089 74.4% Graphite 2% PS-co-PMA 4.4 V NMC 532/ 189 83 75.1% Graphite NMC4.4 V 4.4 V NMC 442/ 193 87 33.8% Control Graphite 0.5% PMA 4.4 V NMC442/ 189 86 81.6% Graphite 2% PS-co-PMA 4.4 V NMC 442/ 189 86 83.1%Graphite

FIG. 4 illustrates the high temperature cycle life testing of anelectrolyte formulation including poly(maleic anhydride) (PMA) as anadditive. The battery included a NMC532 composite cathode and a graphitecomposite anode. The battery was cycled from 2.8V to 4.4V in anenvironment at a temperature of about 40 degrees Celsius.

FIG. 5 illustrates the high temperature cycle life testing of anelectrolyte formulation including poly(maleic anhydride) (PMA) as anadditive. The battery included a NMC442 composite cathode and a graphitecomposite anode. The battery was cycled from 2.8V to 4.4V in anenvironment at a temperature of about 40 degrees Celsius.

FIG. 6 illustrates the high temperature cycle life testing of anelectrolyte formulation including poly(styrene-co-maleic anhydride)(PS-co-PMA) as an additive. The battery included a NMC532 compositecathode and a graphite composite anode. The battery was cycled from 2.8V to 4.4 V in an environment at a temperature of about 40 degreesCelsius.

FIG. 7 illustrates the high temperature cycle life testing of anelectrolyte formulation including poly(styrene-co-maleic anhydride)(PS-co-PMA) as an additive. The battery included a NMC442 compositecathode and a graphite composite anode. The battery was cycled from 2.8V to 4.4 V in an environment at a temperature of about 40 degreesCelsius.

The data presented herein confirm that certain polymer additives canprovide significant improvements to the high temperature capacityretention when added to electrolyte formulations. Specifically,batteries including an NMC composite cathode and graphite compositeanode showed improvements as compared to a comparable battery withoutthe polymer additives in the electrolyte formulation. Notably, theinitial performance of the tested batteries was similar to the controlbatteries, which indicates that the additives do not have a negativeimpact on the cell capacity despite providing improved capacityretention.

While the invention has been described with reference to the specificembodiments thereof, it should be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the true spirit and scope of the invention asdefined by the appended claims. In addition, many modifications may bemade to adapt a particular situation, material, composition of matter,method, or process to the objective, spirit and scope of the invention.All such modifications are intended to be within the scope of the claimsappended hereto. In particular, while the methods disclosed herein havebeen described with reference to particular operations performed in aparticular order, it will be understood that these operations may becombined, sub-divided, or re-ordered to form an equivalent methodwithout departing from the teachings of the invention. Accordingly,unless specifically indicated herein, the order and grouping of theoperations are not limitations of the invention.

What is claimed is:
 1. An electrolyte formulation, comprising: a lithiumsalt, a non-aqueous solvent, and a copolymer additive comprising amaleic anhydride monomer and an aromatic monomer, wherein the copolymeradditive is represented by Formula (f):

wherein m is an integer greater than 1, n is an integer greater than 1,and the copolymer additive has a concentration no less than 0.1 weightpercent relative to a total weight of the electrolyte formulation and nogreater than 10 weight percent relative to the total weight of theelectrolyte formulation.
 2. The electrolyte formulation of claim 1,wherein the copolymer additive has a concentration no less than 0.5weight percent and no greater than 2 weight percent relative to thetotal weight of the electrolyte formulation.
 3. The electrolyteformulation of claim 1, wherein the copolymer additive has a molecularweight of about 9,500 g/mol.
 4. A battery comprising: an anodecomprising an anode active material; a cathode comprising a cathodeactive material; and an electrolyte comprising a lithium salt, anon-aqueous solvent, and a copolymer additive comprising a maleicanhydride monomer and an aromatic monomer, wherein the copolymeradditive is represented by Formula (f):

wherein m is an integer greater than 1, n is an integer greater than 1,and the copolymer additive has a concentration no less than 0.1 weightpercent relative to a total weight of the electrolyte formulation and nogreater than 10 weight percent relative to the total weight of theelectrolyte formulation.
 5. The battery of claim 4 wherein the cathodeactive material comprises nickel, manganese, and cobalt.
 6. The batteryof claim 4 wherein the anode active material comprises graphite.
 7. Thebattery of claim 4, wherein the copolymer additive has a concentrationno less than 0.5 weight percent and no greater than 2 weight percentrelative to the total weight of the electrolyte formulation.